HMGB1 Mediates Macrophage Recruitment and Regional Intervertebral Disc Properties Following Injury

Frequently evaluated in musculoskeletal disease, damage associated molecular patterns (DAMPs) respond to tissue damage and cellular stress by facilitating an inflammatory response via macrophage activation and broad inflammatory pathway activation. In the context of disc degeneration (DD), high mobility group box 1 (HMGB1), a potent intracellular DAMP, is seen to be increased within severely degenerated human IVDs and to directly mediate inflammatory responses within disc cells in vitro. To further understand how HMGB1-mediated inflammation influences DD, this study evaluated the possible protective effect of an HMGB1 knockout on DD pathology following injury. Using a needle puncture injury model in murine caudal IVDs, we evaluated DD pathology within an IVD-specific Hmgb1 knockout (KO) model. Structural and compositional changes in IVD cellularity, histopathology, disc height, and biomechanics were evaluated in addition to an assessment of disc inflammation and macrophage presence throughout the course of degeneration. HMGB1 expression robustly increased shortly following needle puncture injury, and elevated levels were sustained up to 28 days post injury both in injured IVDs and in the IVDs adjacent to the level of injury. IVD-specific Hmgb1 KO mice had an increased disc height following injury both at the injured and adjacent to injury levels compared to wild type (WT) control IVDs. Hmgb1 KO also protected against tissue mechanical property losses at both the injured (dynamic modulus) and adjacent to injury levels (dynamic modulus, creep, and equilibrium modulus) compared to WT IVDs; however, there was no significant effect on histopathologic scores post injury. Hmgb1 KO did not alter basal expression of Ccl2 or Cxcl12 in uninjured discs. Disc puncture injury increased CCL2 secretion; however, these levels were similar in IVDs from WT and Hmgb1 KO mice. Hmgb1 KO reduced sub-acute (28-days post injury) macrophage (F4/80+) recruitment to injured IVDs in vivo. Reduced macrophage migration was also observed in vitro in response to the secretome of an injured Hmgb1 KO IVD compared to injured WT IVD secretome. Overall findings indicate that HMGB1 is upregulated regionally within IVDs, at both the injured level and at the level adjacent to the injury. Results suggest that IVD HMGB1 production plays a role in mediating structural and biomechanical responses of the IVD to injury, particularly in mediating sub-acute macrophage recruitment to the injured IVD.